Method and apparatus for video coding

ABSTRACT

In uniform superposition of two predictions toward a weighted superposition, such as in coding B frames, weighting is based on reliability of a reference picture used for determination of the prediction. Since there is a relationship between the reliability and the respective quantization parameter, coupling to the quantization parameter that is also known in the receiver also can be effected advantageously.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/216,647, filed Aug. 12, 2002, now pending, and is based on and herebyclaims priority to German Application No. 102 254 34.6 filed on Jun. 7,2002, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and an apparatus for video coding, inwhich a predicted picture is produced by averaging reference pictures.

2. Description of the Related Art

Digital video data are generally compressed for storage or transmission,to significantly reduce the enormous volume of data. The compression iseffected both by eliminating the signal redundancy contained in thevideo data and by eliminating the irrelevant signal parts that areimperceptible to the human eye. This is generally achieved by a hybridcoding method in which the picture to be coded is firstly predictedtemporally and the remaining prediction error is subsequentlytransformed into the frequency domain, for example by a discrete cosinetransformation, and is quantized there and coded by a variable lengthcode. The motion information and the quantized spectral coefficients arefinally transmitted.

The performance of the hybrid coding method depends very significantlyon the quality of the temporal prediction. The better this prediction ofthe next picture information to be transmitted, the smaller theprediction error that remains after the prediction, and the lower thedata rate that has to be subsequently expended for the coding of thiserror. A significant task in the compression of video data is obtainingthe most exact prediction possible of the picture to be coded from thepicture information that has already been transmitted beforehand.

The prediction of a picture has been effected heretofore by firstlydividing the picture into regular portions, typically square blocks witha size of 8×8 or 16×16 pixels, and subsequently determining, for each ofthese picture blocks, a prediction from the picture information alreadyknown in the receiver, by motion compensation. In this context, it ispossible to distinguish between two basic cases of prediction:

-   -   unidirectional prediction: in this case, the motion compensation        is effected exclusively on the basis of the previously        transmitted picture and leads to so-called “P frames”.    -   bidirectional prediction: the picture is predicted by        superposition of two pictures, of which one is temporally        preceding and another is temporally succeeding and which leads        to so-called “B frames”. It should be taken into consideration        here that both reference pictures have already been transmitted.

In the last-mentioned case, the actual prediction value must becalculated by averaging from both reference pictures. In the previouslystandardized method MPEG-1 to MPEG-4 or H.263, an equal-weight averagingis always carried out for this purpose, that is to say that the twopossible predictions are added and the resulting sum is then halved.

SUMMARY OF THE INVENTION

An object of the invention is to provide video coding in which the totaldata rate required for the coding of the picture becomes as small aspossible.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 is time diagram of a picture sequence illustrating predictionaccording to the invention;

FIG. 2 a plan view illustrating two successive pictures for explaining asuperposition of blocks that occurs during the prediction;

FIG. 3 is a block diagram for elucidating the apparatus according to theinvention; and

FIG. 4 is a schematic of a computer readable medium storing a programembodying a method for video coding according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The invention essentially resides in a generalization of the uniformsuperposition—known from the coding of B frames—of two predictionstoward a weighted superposition. In this case, the weighting is couplede.g. to the reliability of the reference picture used for thedetermination of the prediction. Since there is a relationship betweenthe reliability and the respective quantization parameter, coupling tothe quantization parameter that is also known in the receiver canadvantageously also be effected.

FIG. 1 illustrates a picture sequence . . . n−2, n−1, n, n+1, n+2 . . .in a time t. In this case, the picture n+1 is e.g. a pureintraframe-coded picture, that is to say a so-called I frame, and thepicture n−1 is, for example, a predictively coded reference picture,that is to say a so-called P frame. Situated between the two picturesn−1 and n+1 is a picture generated by bi-directional prediction, that isto say a so-called B frame. For the case where two or more possiblepredictions are available, here the weights g1, g2, g3, g4 . . . are notnecessarily chosen to be identical, but rather are determined in such away that the best possible overall prediction is effected. This meansthat a uniform weighting for two reference pictures is not necessarilyperformed, rather, as indicated in the block diagram of FIG. 3, each ofthe for example two reference pictures is multiplied by a weightingfactor g1 or g2, respectively, and the two products are added. Theresult is then divided by the sum of the two weighting factors g1 andg2.

In the simplest case, this can be achieved by a pure intraframe-codedpicture n+1 obtaining a higher weight g2 than a predictively codedreference picture n−1 with the weight g1, since intraframe-codedpictures generally have a significantly better quality than predictedpictures.

By way of example, the weight of I frames may be g2=⅔ and the weight ofP frames g1=⅓. In this case, these weighting factors must be transmittedfrom the transmitter to the receiver as additional page information.However, since the quality of the resulting overall prediction issignificantly improved by the adaptation with variable weightingfactors, the disadvantage of the page information that is to beadditionally transmitted is more than compensated for by the saving inthe transmission of the prediction error.

In order to avoid the transmission of the weighting factors and at thesame time also to reduce the complexity in the case of a free choice offactors at the transmitter end, the determination of the weight factorscan also be further formalized. An advantageous refinement is the factthat the factors are inversely coupled to the level width of thequantizers used for the coding of the reference pictures. In concreteterms, this means that a block from a reference picture obtains a highweighting when this reference block has been coded with a lowquantization level, that is to say a high quantization accuracy, andvice versa. The justification for this coupling is grounded in the factthat a large quantization level width or a low quantization accuracy isalso associated with a correspondingly poor picture quality and thus alow reliability as reference picture. Since the quantization levels usedfor each block are transmitted anyway and are thus known to thereceiver, it is possible to directly calculate the weighting factors forthe superposition of the two individual predictions.

FIG. 2 illustrates a block b within the picture n and also four adjacentblocks b1 . . . b4 in the predecessor picture n−1. This makes it clearthat the block b used for the prediction generally simultaneouslysuperposes a plurality of coded individual blocks in the referencepicture n−1 and further definition of the quantization level to be usedis necessary, since, in the general case, the blocks b1 . . . b4 mayeach have different quantization levels. In this case, the weightingfactor g1 can be determined from averaging from the quantization levelsfor the blocks b1 . . . b4 or, for example, by ascertaining the largestquantization level of the quantizers for these blocks.

During the prediction with the aid of weighted averaging, use is made ofat least two reference pictures whose temporal position in the picturesequence is totally arbitrary, in principle, in which case either atleast one reference picture can precede and at least one referencepicture can temporally succeed the predicted picture n or,alternatively, either all reference pictures can temporally precede orall reference pictures can temporally succeed the predicted picture.

FIG. 4 shows a schematic of a computer readable medium 400 storing aprogram 402 embodying a method for video coding according to anembodiment of the invention.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

1. A method for video coding, comprising: assigning different weights to at least two reference pictures situated in an arbitrary temporal position before or after at least one predicted picture of a picture sequence; and calculating the at least one predicted picture by weighted averaging of the at least two reference pictures; wherein said assigning comprises selecting the different weights based on respective reliability of the reference pictures for prediction, further comprising dividing the at least one predicted picture into at least two blocks, wherein said assigning includes forming weighting factors inversely based on quantization level width per block of quantizers used for coding of the reference pictures; wherein a block used for prediction simultaneously superposes a plurality of coded individual blocks in a respective reference picture, and wherein said assigning includes forming a weight of the block used for the prediction either from an inverse average value of the quantization level width of individual blocks or inversely with respect to a largest value of quantization levels.
 2. A method for video coding, comprising: assigning different weights to at least two reference pictures situated in an arbitrary temporal position before or after at least one predicted picture of a picture sequence; and calculating the at least one predicted picture by weighted averaging of the at least two reference pictures; wherein said assigning comprises selecting the different weights based on respective reliability of the reference pictures for prediction, wherein said assigning weights a purely intraframe coded reference picture more highly than a predictively coded reference picture; further comprising dividing the at least one predicted picture into at least two blocks, wherein said assigning includes forming weighting factors inversely based on quantization level width per block of quantizers used for coding of the reference pictures; wherein a block used for prediction simultaneously superposes a plurality of coded individual blocks in a respective reference picture, and wherein said assigning includes forming a weight of the block used for the prediction either from an inverse average value of the quantization level width of individual blocks or inversely with respect to a largest value of quantization levels.
 3. A method for video coding, comprising: assigning different weights to at least two reference pictures situated in an arbitrary temporal position before or after at least one predicted picture of a picture sequence; and calculating the at least one predicted picture by weighted averaging of the at least two reference pictures; wherein said assigning comprises selecting the different weights based on respective reliability of the reference pictures for prediction, further comprising dividing the at least one predicted picture into at least two blocks, wherein said assigning includes forming weighting factors inversely based on quantization level width per block of quantizers used for coding of the reference pictures, wherein a block used for prediction simultaneously superposes a plurality of coded individual blocks in a respective reference picture, and wherein said assigning includes forming a weight of the block used for the prediction either from an inverse average value of the quantization level width of individual blocks or inversely with respect to a largest value of quantization levels.
 4. The method for video coding of claim 3, wherein the different weights are selected from the group consisting of: ⅓, and ⅔. 